Hi, Tried googling on this, but predictably everything is way too complex.
I'm looking for a simple, low speed (few hundred b/s would do it)
self-clocking protocol that I can use to dump debugging data from tiny
micros that are under test. Single line, tolerant to uncertain clock
frequencies and not too complex because the Tx end doesn't have huge amounts
of program memory available. Any suggestions before I go and develop my own?

you can do a lot with a single output pin, a LED and a resistor.
Was what I used for many years to debug micros.

or spit a serial stream at a serial LCD, if you want some sort of dump.
let the serial lcd gobble up most of the hard coding, and memory
overhead. but then, this may not confirm to your uncertain clock
frequencies.

Indeed. Look how some people have got their hands on the firmware for
a device like a camera by managing to load into it their own program
which does nothing more than blink some existing LED indicator. EG
"Hardware-software solution" at
http://chdk.wikia.com/wiki/Porting_the_CHDK (Canon cameras).